It was recognized several years ago that electron beams could be used to delineate structures smaller than those that can be made with UV radiation. The higher resolutions now achievable coupled with the electronics industry's quest to reduce circuit size and increase the switching speed of these circuits, has resulted in better line width control, and circuit chips with minimum feature widths of 1 .mu.m or smaller. The role of the resist in device lithography has become increasingly important since an electron resist must be capable of being patterned by altering its solubility with a defined beam of electrons and subsequently dissolving (developing) the unwanted regions of the resist. Additionally, the resist must protect the underlying substrate during the various etching operations encountered in semiconductor processing. Basically, there are two types of electron resists, which are distinguished by their response upon being exposed to radiation and their resulting solubility behavior in the developing solvent. Materials which are rendered more soluble upon exposure to radiation are termed positive resists. Positive resist action results from radiation-induced degradation of the chain. Contrastingly, negative resists are rendered less soluble upon exposure to radiation and generally negative resist action results from radiation induced cross-linking.
One material which has been utilized advantageously as a positive resist in electron beam lithographic processes is poly(methacrylic anhydride), hereinafter referred to as PMAH. U.S. Pat. No. 4,004,043 (issued Jan. 18, 1977) discloses positive resists made of nitrated polymers and copolymers of methacrylic acid, methacrylic anhydride, etc. Generally, the surface of a semiconductor was coated with a PMAH film prior to exposure to a radiation source such as an electron beam. Many problems arise, however, with the use of a PMAH resist. The solvents typically utilized for PMAH coating, for example, dimethylacetamide, dimethylformamide and N-methylpyrrolidone, do not adequately wet the semiconductor wafer surface to provide a uniform coating, particularly, in the case of silicon.
The PMAH film is usually prepared by first applying a solution coating of poly(t-butyl methacrylate), hereinafter referred to as PtBMA, to the surface of the wafer. The PtBMA is then heated to above 200.degree. C. for two to three hours to convert the coating to a film of PMAH. Additional problems, however, also exist with this method. First, there can be a significant variation in the composition and consistency of the resist produced, thereby making quality control difficult. One explanation is that if there is an incomplete conversion of PtBMA to PMAH, as oftentimes occurs, depending on the unformity of process conditions, certain areas of the film coating will contain both PMAH and PtBMA. During subsequent steps in the lithographic process, the irradiated resist area is dissolved, and thus removed from the wafer surface, upon exposure to the conventional developer solvents. Since the PtBMA on the non-irradiated areas is more soluble in these developer solvents than the PMAH in these same areas, the resist film structure remaining on the wafer surface may be uneven and inconsistent. Another explanation is that crosslinks are formed after the PtBMA is all converted to PMAH.
U.S. Pat. No. 4,508,812 (which issued on Apr. 2, 1985, and which is assigned to the present assignee) discloses a method of fabrication of a PMAH resist. A wafer is first precoated with a thin precursor layer of PtBMA which is then heated to a temperature at which the PtBMA is converted to PMAH, to form a thin, relatively uniform layer of PMAH. A solution of PMAH is then applied over this precursor layer so that a uniformly distributed coating of the desired thickness of PMAH is obtained upon the pre-coated surface of the semiconductor wafer.
There are a number of physical and chemical properties required of resist materials. Solubility is an important consideration in the development of the resist, since films are normally deposited on a substrate from solution by spin coating. Solubility in organic solvents is therefore a necessary requirement. The resist must exhibit etch resistance and it should have adequate adhesion to the desired substrate in order to realize maximum resolution.
Sensitivity, "Q", is a parameter of prime importance. It is conventionally defined as the input incident energy or dose required to achieve the necessary chemical response in the resist. The necessary chemical response is that which on development of the resist results in a faithful replication (in the resist) of the original pattern specified by the circuit designer. Contrast, ".gamma.", another parameter of prime importance, is generally defined as the absolute value of the change in the normalized thickness of the residual resist divided by the change in the log of the incident dose. "Q" and ".gamma." are defined graphically for a positive resist in FIG. 1. ".gamma." is an important parameter because it effects the pattern resolution attainable with a given resist for a given set of processing conditions. ".gamma." can also be more easily defined by reference to the sensitivity curve for a positive resist (FIG. 1), where it is simply the absolute value of the slope of the approximately linear descending portion of the curve.
In the case of a positive resist, the film thickness of the irradiated region remaining after development decreases, until eventually a dose, "D.sub.c ", is reached. This results in complete removal of the film on development. Ths value also represents the sensitivity of a positive resist. The contrast of the positive resist, ".gamma..sub.p ", in an idealized situation, is related to the rate of degradation of molecular weight and is defined as: ##EQU1## wherein "D.sub.o " is the dose at which the developer begins to attack the irradiated film. "D.sub.o " is usually determined in actual practice, however, by extrapolating the linear portion of the plot of the film normalized thickness remaining vs. the log of the dose to a value of 1.0 normalized initial film thickness (refer to FIG. 1).
Many methods have been utilized to attempt to improve the resolution and sensitivity of positive resists. U.S. Pat. No. 3,964,908 (which issued on June 22, 1976) discloses polymers of methyl methacrylate, methacrylic acid, and its anhydride which contain dimethylglutarimide units. Development of images with this type of resist was disclosed utilizing 2-ethoxyethanol(ethyl cellosolve) or in an aqueous ethanol.
U.S. Pat. No. 4,264,715 (which issued on Apr. 28, 1981), however, discloses a method of forming an image on a positive resist layer of poly(methacrylic anhydride) utilizing a developer solvent mixture consisting of a polar organic solvent (capable of dissolving PMAH) and a non-solvent (incapable of dissolving PMAH). Suitable non-solvents for this process included: benzene, toluene and chlorobenzene, methyl isobutyl ketone and methyl ethyl ketone, ethyl acetate, isoamyl acetate, etc. Suitable polar solvents included: dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide.
Given the importance of microcircuit fabrication, a better, method of developing resist images which more easily and reproducibly renders optimum resolution, sensitivity, contrast, etc. is clearly needed. PMAH is clearly an important resist material because of its resolution and sensitivity capabilities. However, finding a developer solution which maximizes these optimum characteristics has previously posed a significant obstacle.